Fix -Wshadow=local warnings

I bumped into some variable shadowings that to my slight surprise the
current warning option -Wshadow=compatible-local does not catch. For
example

const char *p;
char *p;

or

bool skipped;
int64 skipped;

These are not "compatible" in the technical C language sense, but they
are mutually assignable, so IMO just as confusing and fragile.

Also, there are things like

EState *estate;
ExprState *estate;

which are not mutually assignable, but almost as dangerous given the
propensity to cast node types around.

These can be caught if we dial up the warning one notch to
-Wshadow=local. This then flags all shadowing of a local variable by
another local variable. I have fixed all the warnings in the attached
patch. I think everything this catches is obviously bad, so this seems
well worth fixing. (And if we buy into the idea of
-Wshadow=compatible-local, then this is obviously better and more complete.)

So the first patch fixes all the warnings, but doesn't turn up the
compiler flag yet. There is a hiccup with the LLVM headers, because
they themselves trigger these warnings. So the second patch provides a
workaround to silence warnings from those headers. It's a bit different
from what we have done before, but I think it works better for this
case. Alternative ideas welcome. In the third patch, the warning
option is then changed.